The present invention relates to ultrasound imaging of regions associated with cyclic motion. In particular, improved temporal resolution is provided by interleaving ultrasound images from two or more physiological cycles.
The effective frame rate is increased by interleaving frames or images from a plurality of cycles into a single cycle. U.S. Pat. No. 5,099,847 discloses triggering acquisition of frames at different times within different cycles. Once the frames are interleaved, the staggering delays used to trigger in the different cycles provide for an increased effective frame rate. U.S. Pat. No. 5,976,088 discloses increasing the effective frame rate by interleaving frames and filtering across physiologic cycles. For example, the first frames acquired in each of a plurality of cycles are filtered together. The second frames across the same physiological cycles are also filtered together. The process repeats for further frames.
Cardiac motion is adequately cyclic to interleave frames acquired with a frame rate of 15-20 hertz, such as associated with a 50-67 millisecond delay from one frame to the next. Interleaving frames from two cycles may double the frame rate where the frames represent different temporal positions within a cycle in each of the different cycles combined. To avoid overlapping representation of the same temporal location within a physiological cycle, the frames of data for the subsequent or second cycle are delayed by 25-33 milliseconds relative to frames of the first cycle. An R-wave or other trigger event is used to control acquisition timing throughout each cycle. However, delays from the trigger events may result in inaccuracies when high temporal resolution is desired. Cardiac events do not uniformly change so are not easily predictable. 10-20 millisecond variation in the total length of a cardiac cycle may be provided in a healthy individual. The length of successive cardiac cycles can easily vary by 5 to 10%, with variations up to 15% not being uncommon. For example, frame rates on the order of 100-300 Hertz (3-10 milliseconds per frame) are desired for myocardial imaging. Analysis of the deformation rate, also known as strain rate, of the heart requires temporal resolution on the order of 3 to 10 milliseconds. Using a delay from a trigger event may not be sufficiently accurate for interleaving the frames with high temporal resolution. Variations in the cycle and the equipment for detecting the cycle result in a resolution tolerance greater than 3-10 milliseconds per frame.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include methods and systems for improving temporal resolution. Two or more temporal indicators are used for interleaving frames from a plurality of physiological cycles. For example, frames of one cycle are separated from the frames of another cycle based on an electrocardiogram (ECG) signal. Data representing an imaged region for each of the frames is then used for temporally aligning the frames. For example, average velocity waveforms calculated from the frames of each of the cycles are fitted together. The temporal position of each of the frames within a base physiological cycle is determined based on the fitting. The interleaved frames provide data representing the actual physiological cycle with a higher effective frame rate with improved temporal resolution. Course alignment is provided using one event or temporal indicator. A more refined temporal alignment is provided in response to a second temporal indicator.
Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.